1. Field of the Invention
This invention relates to improvements in the manufacture of sealable folding cartons or other containers for food products, consumer goods and the like. More particularly, it relates to a method for improving the gluability of laminated carton blanks having a layer of coated polyester when EVA(ethylene vinyl acetate)-based hot melt adhesives are employed as a sealant for these cartons.
2. Description of the Prior Art
Biaxially oriented polyester film, which exhibits excellent gloss, tensile strength, tear resistant and barrier properties, is frequently used as a substrate for packaging structures which require good barrier protection. The moisture- and gas-barrier properties are further enhanced when a metal layer is applied either by lamination or vacuum metallization. Laminants comprising such metallized polyester bonded to paperboard chipboard are now frequently used to package a wide range of diverse food and consumer products, including dishwashing powder, laundry sanitizer, dentifrice products and powdered soft drinks. Moreover, in the interest of providing tamper evident packaging the major flap can be bonded to the minor flaps of tuck containers.
The surface properties of such commercially-used polyester films provide unsatisfactory printability with inks and unsatisfactory gluability with adhesives commonly utilized in the folding carton industry. As a result, manufacturers of such polyester film have adopted various methods to enhance the ink and adhesive receptivity of their product. Such efforts have included corona discharge treatments of the polyester and application of various coatings to the polyester film. Such coatings include latex resins which impart the high specific surface energy levels necessary for proper wet-out of inks and adhesives, leading to a uniform, continuous film of the applied ink or adhesive. Especially useful are the chemically-primed polyester films sold as Melinex 813 by ICI Americas, Inc. and as Hostaphan 2500 by American Hoechst Corporation. These films are coated on one side with a resinous material designed to enhance printability and gluability.
A typical carton for consumer products based on these polyester coated films is manufactured by the following procedure. The film is metallized on the non-coated side with aluminum via a conventional vacuum metallization technique. The metallized surface is then adhered to paperboard using a casein modified polyvinyl acetate as an adhesive. The coated side of the polyester film is surface printed by a rotogravure process and then overlacquered. The printed product is then die cut into carton blanks. The carton manufacturer's joint is glued on certain unprinted areas with an aqueous copolymer resin adhesive such as, for example, Ajax AC-645-3, available from Ajax Adhesives (Chicago, Ill.). The remaining joints are glued when the carton user performs packaging and filling operations.
With acrylic modified nitrocellulose inks, good ink adhesion is obtained on the chemically coated surface of the polyester film. The chemical coating on this film also enhances, to a certain extent, the gluability of the aqueous copolymer adhesive used in the manufacturer's joint of the folding carton. Although the chemical coating on polyester film such as Melinex 813 and Hostaphan 2500 provides some gluability improvements with respect to aqueous copolymer adhesives, gluability is still unsatisfactory with conventional EVA-based hot melt adhesives that are typically used on the flaps of the cartons joined during packaging and filling operations. Such adhesives are advantageous because their fast setting rate and short compression line requirements permit efficient operation of packaging and filling lines. Due to gluability problems, if one wishes to use a hot melt adhesive, one must resort to rubber-modified adhesives, such as Findley #791-334, available from Findley Adhesives (Elm Grove, Wis.). Such modified adhesives, however, do not "machine" well on conventional packaging and filling machinery. Thus, a need exists for gluability enhancements that work well for conventional EVA-based hot melt adhesives.
Efforts have been directed to developing a satisfactory gluability enhancing composition which could be applied to the appropriate adhering surfaces of polyester-based carton blanks in order to enhance the gluability of EVA-based hot melt adhesives to such blanks. For such development, it is important that the improved gluability be exhibited at a range of ambient temperatures from about 0.degree. F. (-18.degree. C.) to 120.degree. F. (49.degree. C).
A gluability enhancing composition was investigated which consisted of a blend of polyvinyl butyral and polyamide resins in a solvent system consisting of a mixture of ethanol, toluene and aliphatic hydrocarbons; this composition is available under the designation F-92036 from Inmont Corporation (Chicago, Ill.). This gluability coating was applied to the abovementioned metallized Melinex 813/paperboard laminant by a rotogravure process. The gluability coating showed good adhesion to the coated surface of the polyester film and it initially exhibited enhanced gluability with a conventional EVA-based hot melt adhesive (Findley 6301-335). However, bond failure occurred at temperatures of 40.degree. F. (4.degree. C.) and below because of separation of the applied gluability coating and the coated polyester surface.
It is therefore a prime object of this invention to develop a successful method for enhancing the gluability of a resin-coated polyester surface with EVA hot melt adhesives. It is a further object to provide improved polyester film-covered cartons and other containers for foodstuffs, consumer goods and the like which have adequate sealing at ambient temperatures.